The drive for reduced cost per unit wafer processed characterizes the semiconductor industry. The semiconductor industry continuously searches for ways to increase wafer output and/or reduce overall equipment costs (costs of ownership) and production cycle time. Among the factors significantly affecting cost of ownership for a given piece of equipment are clean room costs, footprint, labor costs, and equipment throughput and utilization. It is well recognized that overall semiconductor wafer fabrication tool ("fabrication tool") productivity increases and cycle time reductions are achieved by ensuring a continuous supply of wafers at each fabrication tool. Wafers are transported in the factory in wafer carriers comprising either open cassettes or closed pods of various types. The desire for smaller lot sizes increases the number of wafer carriers circulating in a factory and requires increasing the transaction capability of the factory material handling system.
Conventionally a continuous supply of wafers has been accomplished by employing a local buffer supply (i.e., a supply of wafers at the fabrication tool). Examples of prior art local buffer supplies are manual storage shelves; the "FABS" unit marketed by Applied Materials which comprises a horizontally arranged series of four wafer carriers located in front of a fabrication tool's load lock chamber; and the "MINI BUFFER" marketed by Jenoptik/Infab, Inc. which comprises a vertical buffer positioned next to a fabrication tool's load lock chambers. The Infab MINI BUFFER comprises a series of vertically arranged shelves and one or more adjacent load ports for access by the tool's loader robot, and/or for access by factory transport agents. Conventionally one MINI BUFFER is positioned adjacent to each load lock. The loader robot may then access either MINI BUFFER to obtain a wafer carrier for loading to either load lock.
Although such methods maintain a continuous local buffer supply of wafer carriers, they occupy a considerable amount of floor space, thus increasing the system's overall size and cost of ownership. The fact that fabrication tools are frequently maintained in a clean room environment further exacerbates the increased cost associated with the system's larger footprint. In addition, most prior art systems do not allow simultaneous access by the tool loader and the factory transport agent, and thereby complicate factory transport scheduling. To provide fault tolerance and to enhance continuous flow manufacturing, fabrication tools which perform the same process (or related processes) are often arranged in a collection, yet mechanisms for interconnection of theses collections is seldom provided.
A related, commonly-assigned U.S. patent application Ser. No. 08/763,596, filed Dec. 11, 1996, discloses a Compact Apparatus and Method for Storing and Loading Semiconductor Wafer Carriers. U.S. patent application Ser. No. 08/763,596 provides an apparatus which provides substantial footprint reductions over conventional systems, and which advantageously allows for independent operation of the factory transport agent (i.e., the mechanism that transfers wafers from the factory to the load buffer apparatus' factory load port) and the fabrication tool's loader mechanism, and for local interconnection of fabrication tools. However, a need exists for an apparatus and method which provides these advantages and which is easily retrofitable to standard fabrication tools in use in the industry.